Apparatus and method for decreasing bio-effects of magnetic fields

ABSTRACT

A magnetic field generator includes a power source and a coil connected to the power source to generate a time-varying magnetic field. Energy is applied to the coil so that the coil generates a time-varying magnetic field gradient with a magnitude of at least 1 milliTesla per meter and a rise-time of less than 10 microseconds. One or more of a capacitor, a multi-stage high-voltage switch, and/or a pulse-forming network may assist with the generation of the magnetic field gradient.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This is a United States Non-Provisional Patent Application that reliesfor priority on U.S. Provisional Patent Application Ser. No. 61/074,397,filed on Jun. 20, 2008, the contents of which are incorporated herein byreference.

GOVERNMENT LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Contract No.2R42HL086294 awarded by the “National Lung and Blood Institute”.

FIELD OF THE INVENTION

The present invention concerns an apparatus and a method for decreasingthe bio-effects of magnetic field gradients on tissue(s). Morespecifically, the present invention concerns an apparatus and a methodto decrease bio-effects on neurological tissue from magnetic fieldgradients, such as those experienced during Magnetic Resonance Imaging(“MRI”).

DESCRIPTION OF RELATED ART

As should be appreciated by those skilled in the art, MRI is atechnology whereby a magnetic field gradient is applied to subatomicparticles in tissue to spatially encode a subsequent response from theatoms and molecules in the tissue to a radiofrequency pulse. Afterdetection of an electromagnetic response from the tissue, an image ofthe tissue is generated partly based on that response.

Unfortunately, the generation of magnetic field gradients (e.g.,switching gradients) may elicit undesirable responses in a livingorganism by induction of electrical pulses in nerves and otherelectrically-sensitive tissues.

A model describing electromagnetic bio-effects was described by D RMcNeal and J P Reilly in the mid to late 1970's (D R McNeal, “Analysisof a model for excitation of myelinated nerve,” in IEEE Trans. Biomed.Eng., 23:329-337, 1976; and J P Reilly, “Electric and Magnetic fieldcoupling from high voltage AC power transmission lines—Classification ofshort-term effects on people,” in IEEE Trans. on Power Apparatus andSystems, 97(6): 2243-2252, 1978) based on classic membrane-excitabilityrelations (A L Hodgkin and A F Huxley, “A quantitative description ofmembrane current and its application to conduction and excitation innerve,” in J. Physiol., 117:500-544, 1952).

Subsequent elaborations of the model were disclosed by Reilly in“Sensory effects of transient electrical stimulation: Evaluation with aneuroelectric model,” IEEE Trans. Biomed. Eng., 32(12) 1001:1011, 1985.

The model elaborated by Reilly invokes the telegraph equation firstapplied in the late 1800's to design trans-Atlantic cables.

As in underwater cables, neurons are organized in sections that areseparated by nodes of Ranvier. These nodes enforceresistance/capacitance (“RC”) equations that govern neuronal behavior,with time constants related to neuronal diameter and inter-nodaldistances.

Having provided a brief overview of the electrical characteristics,attention is now turned to magnetic field gradients.

With respect to MRI, magnetic field gradients have at least onecomponent of particular interest: the slew rate. The slew rate refers tothe rate of change in the magnitude of the gradient, which is typicallymeasured in Teslas per meter per second (T/m/s).

As the laws of physics dictate, changes in magnetic fields result in thegeneration of electrical fields. Changes in magnetic field gradients intissue, therefore, also result in the formation of electric fields.

When studying neurological tissues, the changes in magnetic fieldgradients depolarize nerves, once a threshold is reached. This thresholdbecomes higher as the pulse duration becomes shorter. The relationshipbetween these variables follows a traditional, hyperbolic curve.

It is noted that experimental studies in humans (D. J. Schaefer, J. D.Bourland, and J. A. Nyenhuis, “Review of Patient Safety in Time-VaryingGradient Fields,” in J. Magnetic Resonance Imaging, 12:20-29, 2000) havevalidated the basic model for pulse durations as short as fiftymicroseconds.

In response to various MRI studies, regulatory agencies have codifiedthe strength-duration model into law, at least in Europe and the UnitedStates. (See Requirements for the Safety of MR Equipment for MedicalDiagnosis, IEC 60601-2-33 (with respect to Europe); see also Guidelinesfor Premarket Notifications for MR Diagnostic Devices, 21 C.F.R. §807.87(with respect to the US).)

As a result of these studies and as a result of the regulations that arebased on these studies, MRI manufacturers have attempted to designtriangular pulse sequences to conform to the limits prescribed by theReilly model, among others.

One such attempt is described in U.S. Pat. No. 6,198,282, which isdirected to an optimized gradient system for providing minimum-durationgradient pulses, the contents of which are incorporated herein byreference.

The prior art also includes evidence of interest in thestrength-duration curve from another direction, as manufacturers ofneuron-stimulators try to shape the pulse in order to increasestimulation. (See P. J. Maccabee, “Influence of pulse sequence, polarityand amplitude on magnetic stimulation of human and porcine peripheralnerve,” J. Physiol., 513:571-585, 1998).

As a result of numerous studies and advancements, those in the industrytraditionally have applied magnetic field gradients less than athreshold for neuronal stimulation. As a result of the reduced gradientmagnitude, the MRI diagnosis takes longer, and has reduced spatialresolution, than it would otherwise have with a higher magnetic gradientfield strength.

There appears to be wide acceptance in the MRI community in the beliefthat the attainment of high gradient slew rates (i.e., the change inmagnetic gradients over a short period of time) would be a welcomedevelopment for the industry.

As should be immediately apparent, one impetus for increasing slew ratesis to reduce scan time. It is postulated that, if a given MRI sequencerequires a certain number of pulses, then the application of shorter(but stronger) pulses, and of pulses with shorter ramp times, wouldpermit the MRI sequence to be completed in a shorter time period (i.e.,faster) than conventional techniques.

Among other benefits, saving time improves safety for unstable patients.Saving time also may reduce the cost of the MRI sequence.

A second incentive for increasing slew rate is to increase gradientfield strength, which improves spatial resolution. For a givenprescribed pulse sequence, the faster one can ramp up the magnetic fieldper pulse, the higher the gradient strength will be for the same overallscan time. Since the gradient strength is proportional to the spatialresolution of the MRI image obtained, a higher slew rate will result ina better spatial resolution. Increased spatial resolution may improvemedical diagnosis in some cases.

As discussed above, for manufacturers of MRI devices, limits have beenset for slew rates based on studies concerning the presence ofbio-effects due to neuronal stimulation. These limits have placed alimit on currently-available scanning technologies.

As also should be appreciated by those skilled in the art, thegeneration of a magnetic field gradient with a very small durationpresents technological challenges as well. Accordingly, there also hasbeen a technological barrier to decreasing the duration of the magneticfield gradient pulses.

With respect to technological limitations, in some MRI devices, switchesare used to trigger the generation of a magnetic field gradient. Thetypes of switches traditionally used include Insulated-Gate BipolarTransistor (“IGBT”) and Metal Oxide Semiconductor Field EffectTransistor (“MOSFET”)-based devices. However, these traditional switchesare not capable of creating magnetic filed gradients with a sufficientlyshort duration to avoid neuronal stimulation.

Recent developments in switches offer a solution to the technologicalproblem experienced with prior art MRI devices.

Specifically, several generations of plasma physics experimentalistshave led the development of reliable solid-state switches andpulse-forming lines that are just now being introduced into thecommunity. (See H. Sanders and S. Glidden, “High Power Solid StateSwitch Module,” in International Power Modulator Symposium ConferenceRecord, pp. 563-566, 2004).

Those high-power solid-state switches are capable of triggering pulsesof ten-thousand amps in one microsecond, orders of magnitude higher thanthe IGBT and MOSFET-based systems currently employed in commercialgradient field generators for MRI systems (See D. A. Seeber, J. H.Hoftiezer, and C. H. Pennington, “Pulsed current gradient power supplyfor microcoil magnetic resonance imaging,” in Magnetic ResonanceEngineering, 15(3): 189-200, 2002).

It is axiomatic in the field of pulsed power technology that it is ofteneasier to close a switch than to open a switch. To get around thislimitation, solid-state switches may be combined with pulse-forminglines (“PFLs”), which do not require opening switches.

PFLs, which are also known as Blumlein lines (named for the World-WarII-era inventor David Blumlein), are dielectric-filled transmissionlines that begin draining their charge when triggered by a solid-stateswitch. The transmission lines stop delivering current once thedielectric has been drained of charge. Blumlein lines can switch innanoseconds, and maintain currents for milliseconds. (K Gasthaus, “Amillisecond Blumlein line for the power supply of a high power laser,”in J. Phys. E: Instrum., 20:192-195, 1987). In order to deliver pulsesof varying widths, sets of PFLs may be triggered independently of oneanother into a common load.

In view of the foregoing, there exists a desire to apply higher magneticfield gradients to tissue in an MRI environment while avoiding adversebio-effects on that tissue.

SUMMARY OF THE INVENTION

It is, therefore, one aspect of the present invention to provide anapparatus and a method for decreasing the duration of scan times forMRI.

An additional aspect of the present invention involves the applicationof magnetic field gradients with a magnitude greater than thattraditionally employed. In one embodiment, the gradient may be up tofive (5) times greater than previously applied. In other embodiments,the magnitude may be greater.

It is another aspect of the present invention to decrease bio-effectsfrom magnetic field gradients by applying a magnetic field gradient totissue within a time frame below the response threshold for that tissue.

Among other things, the current invention takes advantage of aphysiological loophole: according to accepted physiological models forionic channel transport, bi-phasic pulses on the order of a microsecondare too fast for the nerve to change its polarization state, and are,therefore, effectively ignored. The present invention capitalizes onthis physiological loophole.

According to the model elaborated by Reilly, this loophole implies thatgradient field thresholds may be increased by factors of five or moreabove the usual hyperbolic model without triggering any bio-effects.

The present invention also takes advantage of magnetic pulse deliverysystems that are more powerful than traditional systems used to deliverMRI pulses.

In one contemplated embodiment, the present invention utilizessolid-state switch and/or PFL technologies that conform to therequirements of MRI gradient amplifiers. These requirements includeincreased repetition rates, impedance and trigger matching to MRI pulseprogramming consoles, and electrical/acoustic noise shieldingconsiderations. Downstream modifications include the buttressing ofgradient coils to handle the higher electromagnetic (i.e., J×B) forcesthat will occur at higher current loads, especially in high resolutionMRI systems with high static magnetic fields.

According to classic coil-design codes, eddy current shielding is likelyto be less of a problem than at current regimes, and reductions ininductance are predicted. The influence of eddy currents may further beminimized by acquiring signals at long echo time (“TE”), when eddycurrents have died down, as contemplated by still another embodiment ofthe present invention.

In one further contemplated embodiment, the bore of the cryostat may bemade significantly larger than the gradient coil, further reducing theinfluence of eddy currents.

Other advantages of the present invention will be made apparent from thediscussion that follows and will be appreciated by those skilled in theart.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in connection with the drawingsappended hereto, in which:

FIG. 1 is a schematic illustration of a first embodiment of an apparatuscontemplated by the present invention;

FIG. 2 is a schematic illustration of a second embodiment of anapparatus contemplated by the present invention;

FIG. 3 is a schematic illustration of a third embodiment of an apparatuscontemplated by the present invention;

FIG. 4 is a schematic illustration of a fourth embodiment of anapparatus contemplated by the present invention;

FIG. 5 is a flow chart illustrating one method contemplated by thepresent invention;

FIG. 6 is a graph depicting the magnetic field gradient and durationtypical of a prior art MRI device as used clinically; and

FIG. 7 is a graph depicting the magnetic field gradient and durationcontemplated by the present invention.

DESCRIPTION OF THE INVENTION

The present invention will now be described in connection with one ormore embodiments. It is intended for the embodiments to berepresentative of the invention and not limiting of the scope of theinvention. The invention is intended to encompass equivalents andvariations, as should be appreciated by those skilled in the art.

As a prelude to the discussion of the various embodiments of the presentinvention, a general discussion of MRI devices is now provided. Thisoverview is not intended to be limiting of the invention. It is providedmerely to assist with an understanding of the components of the variousembodiment of the present invention, as detailed below.

As a general rule, an MRI device includes a magnetic field generator,typically a magnetic coil and a radio frequency (“RF”) generator ortransmitter. The magnetic coil generates a time-varying magnetic fieldand the RF generator emits radio waves. The MRI device typically alsoapplies a static magnetic field.

As should be appreciated by those skilled in the art, tissue isprimarily composed of water molecules, each of which contains hydrogenatoms. When a person's tissue is exposed to a strong magnetic field, thehydrogen atoms align with the direction of the magnetic field.Subsequently, the RF generator emits radio waves to the hydrogen atomswhile in the aligned state. Some of the energy from the radio waves isabsorbed by the hydrogen atoms in the water molecules, thereby alteringthe magnetic alignment of the hydrogen atoms. The altered magnetic stateis detected by the detector, which generates signals that are processedto form as an image.

With this overview in mind, reference is now made to FIG. 1. FIG. 1provides a schematic diagram of a first contemplated embodiment of a MRIdevice 10 according to the present invention.

The MRI device 10 includes a power source 12. The power source 12 may beany type of generator suitable for generating power to be provided tothe one or more of the components connected thereto. The generator mayprovide an alternating current (AC) or a direct current (DC), as shouldbe appreciated by those skilled in the art. The precise output of thepower source 10 is not critical to the operation of the presentinvention. Moreover, the power output, once generated, may be converteddifferent types (e.g., AC or DC) as required by individual components ofthe system.

In FIG. 1, the power source 12 is illustrated as providing power to eachof the various components of the MRI device 10 of the present invention.It is noted, however, that the depicted arrangement is meant to beillustrative only. As should be appreciated by those skilled in the art,the individual components of the MRI device 10 may receive power from acentralized source, such as the power source 12. Alternatively, thevarious components may receive power from alternative power sources.Accordingly, the depiction of a single power source 12 is not intendedto be limiting of the invention.

In addition, as detailed below, the MRI device 10 of the presentinvention is illustrated and discussed with reference to singlecommunication lines (or links) extending between the various components.The illustration of single communication lines is meant to simplify thediscussion and illustration of the various embodiments of the invention.As should be appreciated by those skilled in the art, there may bemultiple communication lines between the various components of the MRIdevice 10 as required for their operation. Moreover, the communicationlines are not intended to be limited to wired links. To the contrary,the communication lines may be wireless, as required or desired foroperation of the MRI device 10.

In one contemplated embodiment of the present invention, the powersource 12 may include a plurality of power sources 12, each of whichgenerates power with different characteristics, as required by thedevice(s) and/or components associated therewith.

As depicted in FIG. 1, power from the power source 12 travels in twodirections. Power from the power source 12 is conducted first along acommunication line 14 to a capacitor 16. Power from the power source 12is carried second along a communication line 18 to a processor 20.

The capacitor 16 may be of any size or type as would be appreciated bythose skilled in the art. As is its nature, the capacitor 16 stores acharge based on the power inputted from the power source 12. That chargeis eventually discharged, as discussed in greater detail below.

While FIG. 1 illustrates a single capacitor 16, a plurality ofcapacitors 16 may be employed without departing from the scope of thepresent invention. In one contemplated embodiment, the MRI device 10relies upon a plurality of capacitors 16 for its operation. As should beappreciated by those skilled in the art, plural sets of capacitors 16may be employed to generate successive magnetic field gradients.

In the second flow path, power from the power source 12 is provided tothe processor 20. The processor 20 may be of any type suitable forexecuting instructions, generating data, receiving data, storing data,and the like. In one contemplated embodiment, the processor 12 may be apersonal computer. In other embodiments, the processor 12 may be amainframe computer, a portable computer, a personal data assistant(“PDA”) or any other similar device. The exact design and functionalityof the processor 12 is not critical to operation of the presentinvention. Accordingly, the processor 12 may be of any type suitable forthe operation of the MRI device 10.

The capacitor 16 is connected, via a communication line 22, to a switch,where the term switch refers to one or more high-power solid-stateswitch modules as described above. Accordingly, when the capacitor 16discharges the stored charge, the stored charge passes through thecommunication line 22 to the switch 24.

The switch 24 is connected, via a communication line 26, to a coil 28.Accordingly, when the capacitor 16 is discharged, energy from thecapacitor 16 is passed to the coil 28, which generates a magnetic field30.

The coil 28 need not be a single coil. To the contrary, it iscontemplated that the coil 28 may include a plurality of coils 28, eachof which is capable of generating all or part of the magnetic field 30.Moreover, as should be appreciated by those skilled in the art, whereplural coils 28 are employed, the coils 28 need not be of the same typeor size. To the contrary, it is contemplated that, where plural coils 28are employed, they may be differ from one another to produce magneticfield gradients of differing magnitudes, periods, etc.

As also shown in FIG. 1, the MRI device 10 includes an RF transmitter32. As discussed briefly above, the RF transmitter 32 generates radiowaves 34. While one RF transmitter 32 is illustrated, it is contemplatedthat a plurality of RF transmitters 32 may be employed without departingfrom the scope of the present invention. Moreover, where plural RFtransmitters 32 are employed, they may be of different sizes, types,etc.

As illustrated, the magnetic field 30 and the RF waves 34 are directedat a tissue sample 36. While the tissue sample 36 may be a portion of anorganism, it may also be a complete organism.

After interaction of the magnetic field 30 and the RF waves 34 with thetissue 36, the tissue 36 generates a responsive signal 38 that isdetected by the detector 40. As should be appreciated by those skilledin the art, the signal 38 may encompass a multitude of different signalsfrom the tissue 36. The detector 40 detects the signals 38 and passesthe signals 38 to the processor 20 via the communication line 42. Theprocessor 20 receives and processes the signals 38 to generate an imagerepresentative of the composition of the tissue 36.

As should be appreciated by those skilled in the art, the processor 20may not be the device that processes the signals 38 to generate theimage of the tissue 36. To the contrary, the detector 40 may be combinedwith a suitable imaging device. In still another embodiment, the imagermay be a component separate from the processor 20 and the detector 40.Still further embodiments are contemplated to fall within the scope ofthe present invention.

With continued reference to FIG. 1, the MRI device 10 includescommunication line 42. Communication line 42 is illustrated as a centralbus that connects the processor 20 to the capacitor via communicationline 44, to the switch, via communication line 46, to the coil, viacommunication line 48, and to the RF transmitter, via communication line50. A central bus, however, is not required to practice the invention.To the contrary, multiple connections may be established between thecomponents of the MRI device 10 without departing from the scope of theinvention, as discussed above.

It is noted that the communication lines 14, 18, 22, 26, 42, 44, 46, 48,50 all may conduct data and/or power. The communication lines,therefore, are meant to illustrate multi-modal connections between thevarious components of the MRI device 10. As noted above, each of thecommunication lines 14, 18, 22, 26, 42, 44, 46, 48, 50 may be replacedwith one or more separate connections, as required or desired. Thecommunication lines 14, 18, 22, 26, 42, 44, 46, 48, 50 may beunidirectional or bidirectional as required or desired.

With respect to the communication lines 42, 44, 46, 38, 50, it iscontemplated that the processor 20 will provide operating instructionsto one or more of the components to which it is connected. The processor20, therefore, is contemplated to incorporate control functionality overone or more of the components, as should be appreciated by those skilledin the art. It is also contemplated that controls may be fed from onecomponent to another, as required or desired for operation of the MRIdevice 10.

FIG. 2 illustrates an MRI device 52, which is a second embodimentcontemplated by the present invention. Many of the components of the MRIdevice 52 are the same as illustrated and described in connection withMRI device 10 in FIG. 1. As a result, those components are provided withthe same reference numbers as the components in FIG. 1. Moreover, thedescriptions of these components is not repeated for the sake ofbrevity.

The MRI device in FIG. 2 differs from the MRI device 10 in FIG. 1 in atleast one respect. Specifically, the capacitor 16 and the switch 24 havebeen replaced with a pulse-forming line 54. As should be appreciated bythose skilled in the art, the pulse-forming line 54 may include one ormore capacitors and switches. The pulse forming line 54 is connected tothe power source 12 via communication line 56. The pulse forming line 54is connected to the coil 28 via the communication line 58. The pulseforming line 54 connects to the processor via the communication line 60.

As discussed above, a pulse forming line 54 is also known as a Blumleinline. Pulse forming lines 54 are transmission lines that begin drainingtheir charge in response to a triggering event, such as when triggeredby a solid-state switch. Pulse forming lines 54 also are referred to aspulse forming networks. A pulse forming network (“PFN”) accumulateselectrical energy over a predetermined period of time and releases theelectrical energy in the form of a square pulse in a relatively shortperiod of time, depending upon the materials that make up the PFN 54.PFNs 54 also may be engineered to provide pulsed power. A PFN 54 may becharged by a high voltage power source 12 and then rapidly discharged(possibly via a high voltage switch).

The pulse forming line 54 may be a single line or may be a plurality oflines combined together. The pulse forming line 54 also may be a pulseforming network 54, as discussed above. The exact composition andconstruction of the PFN 54 is not critical to operation of the presentinvention.

FIG. 3 illustrates a third embodiment of an MRI device 62. Thisembodiment is similar to the MRI device 52 illustrated in FIG. 2. Inthis third embodiment, however, a switch 64 has been added between thepower source 12 and the pulse forming line 54. As illustrated, the powersource is connected to the switch via a communication line 66. Theswitch 65, in turn, is connected to the processor 20 via thecommunication line 68. As noted above, the switch 65 may be employed totrigger the pulse forming line 54 to release its energy to the coil 28.

FIG. 4 illustrates a fourth embodiment of an MRI device 70. This fourthembodiment is a modification of the MRI device 62, which is illustratedin FIG. 3. Here, a capacitor 72 has been inserted between the switch 64and the power source 12. The capacitor 72 connects to the power sourcevia the communication line 74. The capacitor 72 connects to theprocessor via the communication line 76. In this embodiment, it iscontemplated that the capacitor 72 will discharge power to the switch64, which will discharge power through the pulse forming line 54 to thecoil 28, as illustrated.

With respect to the embodiments illustrated in FIGS. 1, 2, 3, and 4, andfor purposes of the present invention, the connection between the powersource 12 and the coil 28 is considered to be a controlled communicationline. As a result, the embodiments provide variations for contemplatedconstructions of that controlled communication line.

FIG. 5 illustrates a method 78 contemplated by the present invention.The method 78 starts at 80 and ends at 88. Following the start 80, at82, a magnetic field gradient with a minimum amplitude of 1 milliTeslaper meter (mT/m) is generated. At 84, the magnetic field gradient ismaintained for a least about 1 microsecond and up to about 10milliseconds (this is the plateau time period, as discussed below). At86, the magnetic field gradient is changed in a time frame small enoughto fail to solicit a response from neurological tissue. A decision point85 follows the operation identified at 86. As the flow chart in FIG. 5indicates, via the decision point 85, steps 84 and 86 are repeated asnecessary to obtain physiological and/or anatomic information abouttissues. If the pulse sequence is sufficient to obtain the necessaryphysiological and/or anatomic information about tissues, then the method78 ends at 88.

As discussed in greater detail below, it is contemplated thatapplication of the magnetic field gradient with rise- and fall-times ofless than about 10 microseconds will establish suitable conditions toavoid triggering a biological response from neurological tissue. Asshould be appreciated by those skilled in the art, and as discussed ingreater detail below, the present invention permits the application of amagnetic field gradient higher than that permitted using existingtechnology, because the present invention relies, at least in part onshorter rise- and fall-times than available in the prior art.

FIG. 6 is a graphical illustration of a single magnetic field gradientpulse according to the prior art. The rise-time t_(rise) (approximately150 microseconds) and fall-time t_(fall) (approximately 150microseconds) of the magnetic field gradient pulse exceed a neurologicalresponse time t_(response) for neurological tissue at a typical magneticfield slew rate S_(typical). For a typical clinical system, S_(typical)is 400 T/m/s. Data acquisition is typically performed during the plateauphase, because of difficulties in deconvolving the effects of thechanging gradient field. As a result, the most useful portion of thepulse is the plateau time, t_(plateau). The pulse duration includes boththe rise and fall times as well as the plateau time, for a typical totalduration of 500 microseconds. It should, therefore, be apparent thatshortening the rise and fall times can reduce overall scan time.

As should be appreciated by those skilled in the art, the terms “rise”and “fall” may be applied to both negative- and positive-generatedpulses.

FIG. 7 provides a graphical illustration of a magnetic field gradientpulse generated according to the present invention. The shortened pulserise-time duration t_(short) _(—) _(rise) is less than the rise-timet_(rise) according the practice in the prior art. The current inventiontypically would employ a t_(short) _(—) _(rise) of 10 microseconds orless. The shortened pulse fall-time duration t_(short) _(—) _(fall) isless than the fall-time t_(fall) according the practice in the priorart. The current invention typically would employ a t_(short) _(—)_(fall) of 10 microseconds or less. Also, the pulse ramp times t_(short)_(—) _(rise) and t_(short) _(—) _(fall) are less than the neurologicalresponse time t_(response) for neurological tissue at a typical magneticfield slew rate S_(typical). The slew rate in the present invention isincreased as a result of the reduced pulse ramp times. In the presentinvention, the plateau magnitude is increased, as compared to the priorart, because of several factors. Firstly, the plateau magnitude may beincreased because of the improved switching techniques as describedabove. Secondly, the plateau magnitude may be increased because thetissues are depolarized and repolarized within a short period of timesimilar to the neurological response time t_(response), in amulti-phasic train of pulses as prescribed by the model according toReilly.

As should be apparent, the short duration of the time periods t_(short)_(—) _(rise) and t_(short) _(—) _(fall) make possible the application ofa large magnitude magnetic pulse without soliciting a biologicalresponse from neurological tissue. As discussed above, it is the changein the magnetic field that elicits a biological response. With a rapidchange in magnetic field strength, the effects on the biological tissuemay be minimized such that there is little or no biological responsefrom the tissue.

It is contemplated that the magnitude of the pulse during the timeperiod t_(plateau) may be any value. As discussed above, the magnitudemay be as small as 1 mT/m. It is contemplated that the magnitude may besmaller or greater than 1 mT/m, as required or desired for a particularapplication. While the magnitude of the magnetic field is theoreticallyunbounded at its upper limit, it is foreseeable that the magnitude maybe 1000 mT/m or less.

As for the duration of the time periods t_(short) _(—) _(rise) andt_(short) _(—) _(fall), it is contemplated that these time periods willfall in a range between about 1 and about 10 microseconds. As notedabove, 10 microseconds is a sufficiently short time period in which tochange a magnetic field so that tissue does not react biologically.Naturally, the shorter the duration of t_(short) _(—) _(rise) andt_(short) _(—) _(fall), the smaller the likelihood of eliciting abiological response. In keeping with this premise, it is contemplatedthat the invention will operate with one or both of the time periods,t_(short) _(—) _(rise) and t_(short) _(—) _(fall), being less than about9 microseconds. In another contemplated embodiment, one or both of thetime periods is less than about 8 microseconds. In still anothercontemplated embodiment, one or both of the time periods is less thanabout 7 microseconds. Further, the time periods are less than about 6microseconds in another contemplated embodiment. In one othercontemplated embodiment, one or both of the time periods is less thanabout 5 microseconds. One or both of the time periods may be less thanabout 4 microseconds in yet another contemplated embodiment. Stillfurther, it is contemplated that one or both of the time periods is lessabout than 3 microseconds. In an additional contemplated embodiment, oneor both time periods is less than about 2 microseconds. In one furthercontemplated embodiment, one or both of the time periods is less thanabout 1 microsecond. As should be apparent, the time periods, t_(short)_(—) _(rise) and t_(short) _(—) _(fall), need not be identical induration. The variable t_(short) _(—) _(rise) may be greater than, equalto, or less than the variable t_(short) _(—) _(fall) without departingfrom the scope of the present invention.

In the present invention, the shortening of overall scan times iseffected through at least mechanisms in the present invention: Firstly,the reduced rise- and fall-times lead to an overall reduction in scantime. Secondly, the higher plateau magnitude allows the MRI system toacquire data of quality comparable to the prior art using a shorterplateau time t_(short) _(—) _(plateau).

A comparison between FIGS. 6 and 7 also illustrate one further aspect ofthe present invention. Specifically, the magnetic field gradient asgenerated by the prior art is about 5 times weaker than the magneticfield gradient generated by the MRI device 10 of the present invention.

In summary, the invention consists of an apparatus to deliver very shortmulti-phasic magnetic gradient pulses for magnetic resonance imaging.The invention also includes the method of decreasing bio-effects inmagnetic resonance imaging through the use of reduced pulse lengths andmulti-phasic magnetic gradient pulses.

As discussed, the magnetic pulses are created by releasing electricalcharge stored in capacitors and/or transmission lines into coils and/ortransmission lines near a body part. The coils may include dielectricand/or ferrite materials which assist in the shaping of the magneticpulses. Ferrite materials may be used to construct complex dynamicelectromagnetic fields for circulators and other circuit elementsemployed in microwave transmission and receivers (as disclosed by AnsoftCorporation, in Microwave Journal, June, 1996).

Among other differences, the invention differs from the prior art byprescribing multi-phasic pulse trains within a short period of timesimilar to the neurological response time t_(response), unlike the pulsesequences previously taught.

Other aspects of the present invention should be apparent to thoseskilled in the art based on the discussion provided herein.

What is claimed is:
 1. A magnetic field generator for magnetic resonanceimaging, comprising: at least one power source; at least one capacitor;at least one multi-stage high-voltage switch; at least one coil togenerate a time-varying magnetic field; wherein the stored energy fromthe at least one capacitor is delivered through the at least onemulti-stage high-voltage switch to the at least one coil, so that the atleast one coil generates a time-varying magnetic field gradient with amagnitude of at least 1 milliTesla per meter and with one of a rise-timeof less than 10 microseconds and fall-time of less than 10 microseconds.2. The generator of claim 1, wherein the at least one coil comprises aferrite material.
 3. A magnetic field generator for magnetic resonanceimaging, comprising: at least one power source; at least one switch; atleast one pulse-forming line; at least one coil to generate atime-varying magnetic field; wherein the stored energy from the at leastone pulse-forming line is delivered to the at least one coil.
 4. Amethod of implementing magnetic resonance imaging of structurescontaining neurologic tissue, comprising: generating a magnetic fieldgradient with a minimum magnitude of at least 1 milliTesla per meter;and changing the magnitude of the magnetic field gradient in a durationof less than 1 millisecond, so as to fail to solicit a response fromneurological tissue exposed thereto.
 5. The method of claim 4, whereinthe neurological tissue comprises at least a portion of a livingorganism.
 6. The method of claim 4, where the neurological tissuecomprises at least one neuron.
 7. A method of implementing magneticresonance imaging of structures containing neurologic tissue,comprising: generating a magnetic field gradient with a minimummagnitude of at least 1 milliTesla per meter in such a small timeduration as to fail to solicit a response from neurological tissueexposed thereto; and maintaining the magnetic field at a magnitude of atleast 1 milliTesla per meter for a period of time of at least onemicrosecond; and changing the magnitude of the magnetic field gradientin such a small time duration as to fail to solicit a response fromneurological tissue exposed thereto; wherein the magnetic field isgenerated using a magnetic field generator for magnetic resonanceimaging that comprises at least one power source, at least one coilconnected to the at least one power source to generate a time-varyingmagnetic field, at least one capacitor connected to the coil, whereinthe at least one capacitor stores energy from the at least one powersource that is applied to the at least one coil, and at least onemulti-stage high-voltage switch connected between the capacitor and thecoil, wherein the stored energy from the capacitor is delivered to theat least one coil so that the coil generates the magnetic fieldgradient.
 8. A method of implementing magnetic resonance imaging ofstructures containing neurologic tissue, comprising: generating amagnetic field gradient with a minimum magnitude of at least 1milliTesla per meter in such a small time duration as to fail to solicita response from neurological tissue exposed thereto; and maintaining themagnetic field at a magnitude of at least 1 milliTesla per meter for aperiod of time of at least one microsecond; and changing the magnitudeof the magnetic field gradient in such a small time duration as to failto solicit a response from neurological tissue exposed thereto; whereinthe magnetic field is generated using a magnetic field generator formagnetic resonance imaging that comprises at least one power source, atleast one coil connected to the at least one power source to generate atime-varying magnetic field, and at least one pulse-forming networkconnected between the at least one power source and the at least onecoil, wherein the stored energy from the at least one pulse-formingnetwork is delivered to the at least one coil so that the coil generatesthe magnetic field gradient.